The present invention concerns hydrophobic, self-cross-linkable resins for use in cathodic electrodeposition of films on conductive substrates, to electrodeposition baths containing the same, and to a method of preparing such resins. The present invention also concerns a method for making low temperature curable cross-linked coatings from the material of the invention.
Electrodepositable resin compositions are of course well known in the art. For example, U.S. Pat. No. 4,031,050 discloses cationic electrodepositable compositions of blocked organic polyisocyanates and an amine adduct of an epoxy resin. As disclosed in this patent, electrodeposition of such compounds, which may optionally contain a catalyst for urethane formation, can be carried out to provide coatings on a conductive substrate, which coatings have desirable properties. In this regard, see also U.S. Pat. Nos. 3,984,299 and 4,031,050. However, isocyanate compounds are toxic and highly reactive, requiring the taking of suitable precautions in handling and storing the same.
U.S. Pat. No. 4,017,438 discloses an epoxy resin-derived, cationic electrodepositable resin enhanced by the incorporation of primary amine groups into the resin molecule, by reacting certain polyamine compounds having primary amine groups blocked by ketimine. The ketimine groups when contacted with water, will decompose to provide primary amine functionality as disclosed in this patent. Capped isocyanates are disclosed in combination with the amine-resin adduct to provide, together with a suitable catalyst a cationically electrodepositable resin system. The electrodeposited coating, upon being heated to an elevated temperature, usually in the presence of a cross-linking catalyst, undergoes cross-linking through urethane, hydroxy and amino groups.
As well known, the "capped" or "blocked" isocyanates react with hydroxyl groups and amino groups under conditions of elevated temperature to form urethane and urea cross-linkages.
Numerous literature references exist showing the reactions of primary and secondary amines with, for example, propylene carbonate to yield corresponding hydroxypropyl carbamates (Compt. rend. 1142, 1954). The literature also shows that bishydroxyalkyl carbamates derived from corresponding diamines have been further self-condensed, or transesterified with other diols, to produce linear thermoplastic polyurethanes. For example, see the article "The Preparation of Polymeric and Cyclic Urethans and Ureas from Ethylene Carbonate and Amines" by Elizabeth Dyer and Harvey Scott, J.A.C.S. (1956) pp. 672-675. See also the report .-+.Polyurethane elastomers obtained without the use of diisocyanates" by L. Ya. Rappoport, G. N. Petrov, I. I. Trostyanskaya and O. P. Gavrilova in International Polymer Science and Technology, 8, No. 1, 1981 and an article by Richard D. Cowell entitled: "Thermoplastic Polyurethane Elastomers: Chemistry Properties and Processing for the 80's" in the Journal of Elastomers and Plastics, Vol. 14, (October, 1982) pages 195-203.